What are the classifications of expansion joints? First distinguish between metals and non-metals
Many customers ask "What are the classifications of expansion joints?" In fact, the roughest way to divide them is to look at the materials-metal and non-metal. Metal expansion joints are seen most often, likeUniversal corrugated expansion joint、Corrugated expansion joint for power station industry、Large diameter thick wall expansion jointThe bellows inside are all stainless steel or alloy materials, which are hard-core in pressure and temperature resistance. The same cannot be said for non-metallic expansion joints, such asNon-metallic expansion joint (fabric fiber expansion joint)AndRectangular non-metallic expansion jointThe main body is fiber fabric plus silicone coating, which is mainly used in the occasion of low pressure, large cross section and violent temperature fluctuation of smoke duct.
These two are not substitute relationships. Metal can carry pressure and temperature, but the reaction force is large when compensating displacement; Non-metal has good flexibility and corrosion resistance, but weak pressure bearing capacity. You have to take metal to do non-metal work, and the cost goes up; Taking non-metals to top the high temperature and high pressure is looking for trouble. If the material layer is not selected correctly, no matter how advanced the structure is, it will be useless.
According to the structure: the respective doorways of bellows, sleeve and rotary compensator
The structure determines how the expansion joint absorbs the displacement.Bellows typeIs the absolute mainstream, compensated by ripple deformation. There are many patterns in this, there are single, duplex, with hinges, and with pull rods. LikeCompound hinge transverse expansion joint、External pressure single axial expansion jointIt's all based on this logic.
Sleeve type pipe expansion jointIt relies on the relative sliding of the inner and outer sleeves to compensate the axial displacement, has simple structure and low cost, but the sealing performance is not as good as that of bellows, and is suitable for those pipelines with unclean medium and low pressure.Rotary compensatorIt is another way, which absorbs the axial displacement by converting the axial displacement into rotating motion through the cooperation of a pair of rotating cylinders in the bending section of the pipe. It is especially used in long-distance conveying of steam pipe network, with large compensation amount and small pressure drop.
When choosing a structure, it depends on the direction and space of the pipe. If there are many straight pipe sections, choose axial type; if there are elbows, consider hinge type or rotary compensator. Where the space is compact, the sleeve type is easy to install-it doesn't mean that the more advanced the more suitable, it is the last word that it can be installed and compensated in place.
According to the compensation function: axial, transverse, angular and pressure balance type
This level of classification is the real test of selection skills.
Axial typeThe compensator mainly absorbs the axial expansion and contraction of the pipe, such asUniversal corrugated expansion jointIt is a typical axial product. Several are arranged on straight pipe sections, so thermal expansion and contraction are not afraid.Transverse typeThe good thing is that it absorbs displacement perpendicular to the axis of the pipe,Compound hinge transverse expansion jointThat's what we do.Angular typeWell, with a combination of hinges, the pipe swings at a certain angle, likeDouble hinge expansion joint for air-cooled island vacuum pipelineIt is particularly useful in air-cooled islands with large displacements and multi-directional changes.
If the pressure in the pipeline is high and the blind plate is strong, the ordinary axial compensator will transmit the pressure thrust to the fixed support, which must be made as thick as the city wall. That's when you have to use itStraight pipe pressure balanced expansion jointOrCurved tube pressure balance expansion joint。 Its principle is that a balancing bellows is built in, which eats the blind plate force itself, and the main fixing bracket basically does not bear extra thrust. There are more complicatedCompound straight pipe bypass pressure balanced expansion jointWhich is suitable for the working condition that requires a large compensation amount and does not want the bracket to be stressed. Do you think this classification is important or not? Wrong choice, pipeline vibration, bracket deformation, later is all trouble.
According to the working conditions of the industry: special goods such as power stations, cement and desulfurization
As soon as the industry working conditions come up, the expansion joint has to be "customized". Smoke air duct of power station boiler, high temperature, large cross section, generally usedMetal rectangular expansion jointOrNon-metallic expansion jointWhich not only absorbs thermal displacement but also reduces vibration. Of the cement industryMetal Corrugated Expansion Joints in Cement Industry, that has to be wear-resistant, the dust particles at the head and tail of the kiln are fast, and the material and wall thickness of the bellows have to be increased.
The desulfurization system is more special. The flue gas contains sulfide, and the temperature changes greatly. Ordinary metal bellows can't bear corrosion, so it is commonly usedPTFE compensatorOr a metal hose lined with PTFE, such asPTFE-lined hose。 Also matched with the expansion jointDesulfurization flue gas baffle door、Flue gas baffle doorThese valve equipment have to be considered together when selecting, otherwise the connection size will not match, and the scene will be blinded.
Quick locking categories? Look at these key parameters
Then how do you quickly judge which category to use? Don't rush, just three steps.
The first step is to look at the medium and temperature. High-temperature flue gas and corrosive gas, directly leaning in the direction of non-metal or tetrafluorine lining; High temperature and high pressure steam, honestly choose metal corrugated expansion joint.
The second step is to calculate the compensation amount. Is the axial displacement large? Is there any lateral displacement? If there are multiple directions, preference is given to hinged type or rotary compensators. Like a direct pipeline, then you have to goDirect buried (fully buried) type expansion jointThis goods is directly buried in the soil, and there is a protective sleeve on the outside, so you can't make do with ordinary expansion joints.
The third step is to look at the pipeline constraints. If the position of the bracket has been set, it is necessary to choose the pressure balance type according to the thrust that the bracket can bear. Two days ago, I met a customer who was doing a steam pipe network project. When I came up, I asked, "What are the classifications of expansion joints?" I asked him how to arrange the bracket, and he looked confused. You have to understand that classification is not just a difference in name, each structure corresponds to different force models and installation conditions.
The classification table looks simple, but there are many things involved in the actual selection. The four dimensions of material, structure, function and working condition are cross-combined, and dozens or hundreds of schemes can be discharged. But as long as you grasp the core-the temperature and pressure of the medium, the magnitude of the displacement direction, and the force constraints of the pipeline-you won't deviate. Understanding these, the expansion joint classification is no longer a table, but a selection map in your hand.